Photodynamic therapy (PDT) is a therapy employed routinely in the treatment of superficial dermatological malignancies and is under investigation for a range of additional tumor types. Most applications of PDT involve the use of an active compound, known as a photosensitizer, and a light source, the wavelength of which can be chosen to be appropriate for exciting the photosensitizer.
Photochemical reactions initiated by excited photosensitizers are crucial to their efficacy because these reactions are what lead to apoptosis, or cell death, following light activation. Photosensitizers can initiate these reactions primarily in two ways. Following the absorption of light energy, the excited photosensitizer can either directly react with a biomolecule (Type I photosensitization) or it can transfer its energy to molecular oxygen. Energy transfer to molecular oxygen forms highly reactive oxygen species, such as singlet oxygen, which can then go on to damage biomolecules (Type II photosensitization). This leads to the destruction of any tissues which have either selectively taken up the photosensitizer or have been locally exposed to light.
For example, a PDT treatment of human skin cancer may involve the following steps. First, a photosensitizer is administered to the patient. The photosensitizer is taken up by the cells. The area to be treated is then exposed to light of the appropriate wavelength. The photosensitizer can absorb light and reacts with nearby tissue oxygen, resulting in reactive oxygen species. These reactive oxygen species react with biomolecules, fatally damaging some of the cells in the treatment area.
PDT has been used in the treatment of dermatological tumors where light can be readily applied to the surface of the skin; clinically substantial subsets of skin tumors are difficult to treat by conventional therapies (because of size, site or multiple lesions presentation). In the treatment of skin conditions, the photosensitizer or photosensitizer precursor can be applied topically, and locally excited by a light source. In the local treatment of internal cancer cells, on the other hand, photosensitizers or photosensitizer precursors can for example be administered intravenously and light can be delivered to the target area using endoscopes and fiber optic catheters. Compared to normal healthy tissues, most types of cancer cells are especially active in both the uptake and accumulation of photosensitizers, which makes cancer cells especially vulnerable to PDT, since having more photosensitizer present in a cell leads to more damage to that cell during PDT.